A polymorphic system related to but genetically independent of the chicken major histocompatibility complex.

Analyses of the major histocompatibility complex (Mhc) in chickens have shown inconsistencies between serologically defined haplotypes and haplotypes defined by the restriction fragment patterns of Mhc class I and class II genes in Southern hybridizations. Often more than one pattern of restriction fragments for Mhc class I and/or class II genes has been found among DNA samples collected from birds homozygous for a single serologically defined B haplotype. Such findings have been interpreted as evidence for variability within the Mhc haplotypes of chickens not detected previously with serological methods. In this study of a fully pedigreed family over three generations, the heterogeneity observed in restriction fragment patterns was found to be the result of the presence of a second, independently segregating polymorphic Mhc-like locus, designated Rfp-Y. Three alleles (haplotypes) are identified in this new system.